Cell surface adenosine receptors mediate either stimulating or inhibition of adenylate cyclase activity, and the receptors that mediate these different responses can be discriminated with selected adenosine analogs. 5'-N-ethyl-carboxamide-adenosine is a more potent agonist at stimulatory receptors (Ra) than is N6-phenylisopropyl-adenosine, whereas the reverse potency order is seen with inhibitory receptors (Ri). The potency of adenosine is intermediate between these two analogs. The relative potencies of adenosine receptor agonists is maintained in physiological response in intact cells, such as steroidogenesis and inhibition of lipolysis. As with adrenergic receptors, subclasses of adenosine receptors differ functionally and pharmacologically.